This map shows the overall rate of population change in each
county over that year. The then-booming oil-rich counties of
Texas and western North Dakota drew a huge number of people in.
Most of California and the big cities of the Sunbelt also
continued their long term growth trends:

Business Insider/Andy
Kiersz, data from US Census Bureau

Here are the fifteen fastest growing counties in the country.
Nine of them are in North Dakota or Texas:

Business Insider/Andy Kiersz, data from US
Census Bureau

And on the other side of the coin, here are the fifteen counties
that saw the biggest drops in population between 2013 and 2014: